There are a number of ways to implement Layer 2 Virtual Private Networks (L2VPN), including Virtual Private LAN Service (VPLS), Provider Backbone Bridge (PBB), as well as dual-stack VPLS/PBB networks and so on.
In L2VPN, a Pseudo Wire (PW) is a virtual point-to-point connection over a communication network, which may be established between two Provider Edge devices (PE) by maintaining a Virtual Switching Instance (VSI).
In a Hierarchical Layer 2 Virtual Private Network (H-L2VPN), a main PW and a backup PW may be provided to a node to increase the reliability of connectivity via PW links. The main PW is used as the preferred forwarding path. When the main PW fails, traffic may be diverted to the backup PW to maintain the service.